gh-122917 Allow stable API extensions to include a multiarch tuple in the filename

This permits stable ABI extensions for multiple architectures to be
co-installed into the same directory, without clashing with each other,
the same way (non-stable ABI) regular extensions can.

It is listed below the current .abi3 suffix because setuptools will
select the first suffix containing .abi3, as the target filename.
We do this to protect older Python versions predating this patch.

Author: stefanor

Doc/c-api/stable.rst

@@ -114,7 +114,8 @@ On Windows, extensions that use the Stable ABI should be linked against
 ``python39.dll``.
 
 On some platforms, Python will look for and load shared library files named
-with the ``abi3`` tag (e.g. ``mymodule.abi3.so``).
+with the ``abi3`` tag (e.g. ``mymodule.abi3.so`` or
+``mymodule.abi3-x86-64-linux-gnu.so``).
 It does not check if such extensions conform to a Stable ABI.
 The user (or their packaging tools) need to ensure that, for example,
 extensions built with the 3.10+ Limited API are not installed for lower

Doc/whatsnew/3.15.rst

@@ -284,6 +284,14 @@ Other language changes
   not only integers or floats, although this does not improve precision.
   (Contributed by Serhiy Storchaka in :gh:`67795`.)
 
+* Stable ABI extensions may now include a multiarch tuple in the
+  filename, e.g. ``foo.abi3-x86-64-linux-gnu.so``.
+  This permits stable ABI extensions for multiple architectures to be
+  co-installed into the same directory, without clashing with each
+  other, as regular dynamic extensions do. Build tools will not generate
+  these multiarch tagged filenames, by default, while still supporting
+  older Python versions that don't recognize these filenames.
+  (Contributed by Stefano Rivera in :gh:`122931`.)
 
 New modules
 ===========

Makefile.pre.in

@@ -1958,6 +1958,7 @@ Python/interpconfig.o: $(srcdir)/Python/interpconfig.c $(srcdir)/Python/config_c
 Python/dynload_shlib.o: $(srcdir)/Python/dynload_shlib.c Makefile
 	$(CC) -c $(PY_CORE_CFLAGS) \
 		-DSOABI='"$(SOABI)"' \
+		$(MULTIARCH_CPPFLAGS) \
 		-o $@ $(srcdir)/Python/dynload_shlib.c
 
 Python/dynload_hpux.o: $(srcdir)/Python/dynload_hpux.c Makefile

Misc/NEWS.d/next/C_API/2024-08-12-09-48-04.gh-issue-122931.QwHc2l.rst

@@ -0,0 +1,2 @@
+Allow importing stable ABI C extensions that include a multiarch tuple
+in their filename, e.g. ``foo.abi3-x86-64-linux-gnu.so``.

Python/dynload_shlib.c

@@ -45,6 +45,9 @@ const char *_PyImport_DynLoadFiletab[] = {
     "." ALT_SOABI ".so",
 #endif
     ".abi" PYTHON_ABI_STRING ".so",
+#ifdef MULTIARCH
+    ".abi" PYTHON_ABI_STRING "-" MULTIARCH ".so",
+#endif
     ".so",
 #endif  /* __CYGWIN__ */
     NULL,